The invention relates to a method for reaching a neutral position located between two end positions of a gearshift fork in a shift transmission of a motor vehicle.
From the EP 1 300 614 A2 a method for finding a neutral position of a gearshift fork is known. The gearshift fork can be moved between two end positions in two directions. If the gearshift fork is positioned in one of its end positions, a gear of the shift transmission is engaged. For engaging a gear, starting from the neutral position, the gearshift fork passes through a synchronization point related to the particular gear, wherein the rotational speed of an input shaft of the shift transmission is synchronized with the rotational speed of an output shaft. After passing beyond the synchronization point, a positive interlocking is performed between a loose wheel that relates to the respective gear and a shaft (input shaft or output shaft) supporting the loose wheel. By means of this positive interlocking between the loose wheel and the respective shaft the gear is engaged.
According to the EP 1 300 614 A2 a latching force acts upon the gearshift fork or the shifting rod that is part of the gearshift fork, said latching force urging the gearshift fork in the direction of the neutral position when closer to the neutral position, and away from the neutral position when the gearshift fork is more remote from the neutral position. The method according to the EP 1 300 614 A2 focuses on finding the neutral position when the gearshift fork is already close to the neutral position and the latching force pushes the gearshift fork in the direction of the neutral position. For finding this approximate neutral position the EP 1 300 614 A2 suggests to determine the two end positions of the gearshift fork or the two end positions of a shifting sleeve that are moved by means of the gearshift fork and to determine by calculating the arithmetic mean value of the distance between the end positions the neutral position that is typically provided in the middle between the end positions. This, however, requires at least one sensor for determining the position or the position change of the gearshift fork.
In case of failure of the sensor S4 in determining the position, the method according to the EP 1 300 614 A2 can no longer be performed. It is no longer possible to find the neutral position reliably.